Why was knowledge about the past in the form of myths and fairy tales in the preliterate era?

Because then all knowledge was passed from mouth to mouth. People sometimes embellished events, in connection with which, they began to be called legends and myths. On the one hand, they reveal the events of the past, but on the other hand, they do not have 100% historical accuracy, since the events that are described there tell about supernatural forces. Only with the advent of writing did more reliable sources appear.